Little is known about clinical predictors of aortic dissection in children and young adults, although many children with certain genetic conditions have significant aortic dilation, and aortic dissection has been reported as early as 6 months of age. Due to the paucity of data available in children and young adults, management of these patients is widely variable, and optimal timing of prophylactic aortic surgery is unknown, despite frequent cardiac imaging to assess aortic size. Standard measures of risk in aortic dilation such as absolute aortic dimensions and rate of aortic dilation may not apply in growing children, and there is little data to show at what threshold to manifest concern. Vascular tortuosity is a common finding in connective tissue disorders, especially due to single gene mutations. We have identified a new potential imaging biomarker, the vertebral artery tortuosity index (VTI), that in pilot research appears to be an independent indicator of risk for aortic dissection and other adverse cardiovascular events in children and young adults. Our primary hypothesis is that the vertebral artery tortuosity index (VTI) is a stable imaging biomarker that can facilitate the prospective identification of children and young adults with thoracic aortic aneurysm that are at high risk of adverse cardiovascular events, in combination with other known risk factors. To test this hypotheses, our specific aims are: 1) To compare the distribution of VTI among diagnostic groups and among demographic characteristics; 2) To confirm the ability of VTI to predict adverse cardiovascular outcomes including rapid rate of aortic dilation, aortic surgery, aortic dissection, and death; and 3)To determine if increased VTI is associated with coarctation of the aorta, leaflet fusion pattern, or presence of aortic dilation among patients with bicuspid aortic valve. The overall intent of this research is to better understand risk factors for aortic dissection and death in children and young adults with thoracic aortic dilation, to aid in development of new diagnostic and treatment therapies to improve outcomes in these patients, and to develop better criteria for providers to determine optimal timing of prophylactic aortic surgery. The results of this research can potentially help providers more appropriately monitor and treat patients at high risk of aortic dissection which can prevent death in this population. This study can also potentially result in decreased high cost of serial imaging for individuals at low risk of aortic dissection and adverse cardiovascular events.